Expose extendedProperties of Google Events to the API

[jpmelos]

Return Google's extendedProperties from events.

[jpmelos]

@mrhiggi-close after having given it some more thought, I don't want to merge this. 😅

Maybe a more appropriate solution here would be to extract "metadata" from the events when they are synced into sync-engine (in AbstractEventsProvider.sync_events), and than expose that as a more provider-agnostic field metadata. But it starts to complicate the implementation: we'll need a SQL migration to add the new metadata field to Event. Then, in GoogleEventsProvider.sync_events we can get metadata from extendedProperties, and in MicrosoftEventsProvider.sync_events we can get them from the description (or use singleValueExtendedProperties if we like to suffer).

But this implementation here in this PR is feeling more and more wrong the more I think about it. It's adding something that is very Google-specific in a provider-agnostic interface, since it's just copying extendedProperties directly from Event.raw_data and putting it in the API payload. Sounds like a decision that will force us into making even worse decisions down the road.